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ns7h
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« on: March 06, 2023, 03:33:56 PM »

I have an interesting problem that I am sure has a practical explanation.  I got a Harvey Wells TBS-50C Senior Bandmaster without a power supply.  I put together a supply that would provide 6.3 VAC winding and approximately 450 VDC for B +.  I used silicon diode rectifiers for the B+ supply and left the 5 Volt rectifier filament winding floating since it was not needed.  The transformer was out of a hamfest Globe Scout but was not the original transformer.

The power supply provides the nominal values needed under load and the  transmitter functions and provides about 20 watts RF and good audio.

Nominal 6.3 VAC heater windings are at spec with an input voltage (from a Variac) of 95 VAC!!  I have a 110 VAC TR relay that receives the actuation voltage through the power supply and 95 VAC is too low to keep engaged.  I replaced the transformer in my Globe Scout with one with a higher HV winding thus used the old one in the supply.  I did not see with either transformer operating in the Globe the high heater voltages for 6.3 VAC at 115 VAC line supply.  The tube complement is different, but adding the loads together in either transmitter shows similar 6.3 VAC loads  3.5 total Amps vs 3.1.

The significant difference is that in one, the 5 VAC winding is used with approximate 3.4 amps load ,l the TBS-50 supply has this floating.

Without a dissertation of equivalent circuits, mutual conductance, etc., does the reduced load on the transformer due to a floating 5 volt filament winding effect the 6VAC winding significantly?

The field fix is "in the mail" with some .1 ohm , 5 watt resistors to drop approximately a volt at load and at 110 VAC line voltage

I haven't seen this effect in my other tube rigs.

Thanks...Bob, NS7H
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K9MB
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« Reply #1 on: March 06, 2023, 08:20:30 PM »

One way to lower the output of an older transformer made for 110-115vac is to connect the 5 volt filament winding in series with the primary.
This will make the primary have more turns, so the ratio from primary to secondaries will also rise and that will lower all secondary voltages a bit-maybe about 4%?
The risk is that the 5 volt winding cannot handle the 115 volts, but given that those windings were for cathode filament rectifiers which had hundreds of volts DC on them, it might be safe.
The polarity of the series connection should be checked to see which way works best.
On secondaries, a bucking connection works well to lower the output voltage, but on the primary, maybe you just want to increase the primary to secondary ratio…
The primary to 6.3 volt ratio is likely 18-19/1.
You may find that the voltage you would get at 115 will be the same as at 120volts.
Of course, you could just wire in a 5U4 and maybe the thing solves itself.
Going from thermionic rectifiers to silicon is not as simple as just soldering in the 1N5408s or whatever. Tube rectifiers are inherently gentle on circuits and capacitors with far lower inrush currents…
Considering you’re restoring a legacy radio, the least risky thing is to keep it all in the old school tech from the 30s-60s…😉
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ns7h
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« Reply #2 on: March 06, 2023, 09:34:03 PM »

Thanks for the reply.  My question was if the 5 volt winding is floating and not loaded affects the loading of the other windings.   The power supply's 6.3 volt winding from a transformer with normal, nominal 110 volt primary input under load should be in the ball park of 6.3 volts instead of greater than 7 + volts.  I understand that a tube rectifier replaced by silicon affects HV DC levels. HOWEVER, I am only concerned to keep the filament voltage within range for the tube requirements which is approximately 6.3 volts at 4 amps.  At 95 volts AC input to the primary windings of the transformer from the rheostat I get 6.3 VAC in the secondary filament winding  under load.  At 110 VAC measured at the rheostat input I get greater than 7 volts AT load for the 6.3 volt filament again AT LOAD.  A field fix to have a series dropping resistor (~ 1 volt) around .1 ohms, 5 watts should allow for a more appropriate filament voltage at 110 VAC primary input to the transformer.  The transformer used in this supply was taken from a similar application (WRL Globe Scout) that utilized the 5 volt winding for the rectifier did not have this excessive filament (6.3 VAC winding) voltage at a 110 VAC primary winding input.

Thus, I have the fix (I think), but don't understand the problem that is indicated by an excessive filament voltage at load with a similar tube loading application unless there is interaction with the 5 volt filament floating instead of a loaded secondary HV winding tube rectifier tube.  Again a little knowledge is dangerous, and I have a little....

Thanks again for the reply. 73 , NS7H

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Patrick J. / KD5OEI
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« Reply #3 on: March 06, 2023, 09:43:06 PM »

unloading the 5V winding will not cause the voltage mismatch you are seeing.

What does the 5V winding voltage measure compared to the 6.3V winding?


you might have one of:

transformer has a multi tap primary with a 100V selection

transformer is from a global region where 100VAC is the line voltage.

transformer is some special type expecting a resistor or regulator in series with the primary.

transformer is expecting a resistance in series with the 6.3V filaments, such as some sort of ballast, etc.
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Radio Candelstein - Flagship Station of the NRK Radio Network.
K9MB
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« Reply #4 on: March 07, 2023, 12:03:26 AM »

Thanks for the reply.  My question was if the 5 volt winding is floating and not loaded affects the loading of the other windings.   The power supply's 6.3 volt winding from a transformer with normal, nominal 110 volt primary input under load should be in the ball park of 6.3 volts instead of greater than 7 + volts.  I understand that a tube rectifier replaced by silicon affects HV DC levels. HOWEVER, I am only concerned to keep the filament voltage within range for the tube requirements which is approximately 6.3 volts at 4 amps.  At 95 volts AC input to the primary windings of the transformer from the rheostat I get 6.3 VAC in the secondary filament winding  under load.  At 110 VAC measured at the rheostat input I get greater than 7 volts AT load for the 6.3 volt filament again AT LOAD.  A field fix to have a series dropping resistor (~ 1 volt) around .1 ohms, 5 watts should allow for a more appropriate filament voltage at 110 VAC primary input to the transformer.  The transformer used in this supply was taken from a similar application (WRL Globe Scout) that utilized the 5 volt winding for the rectifier did not have this excessive filament (6.3 VAC winding) voltage at a 110 VAC primary winding input.

Thus, I have the fix (I think), but don't understand the problem that is indicated by an excessive filament voltage at load with a similar tube loading application unless there is interaction with the 5 volt filament floating instead of a loaded secondary HV winding tube rectifier tube.  Again a little knowledge is dangerous, and I have a little....

Thanks again for the reply. 73 , NS7H



Ok, I believe I understand your concern.
As Patrick says, the soaring voltages cannot be explained by the lack of load on the 5 volt winding, though it will have a small effect. My point was that you could harness the 5volt winding to reduce the 6 volt winding by adding tue five volt winding to the primary winding. The 5 volt winding will become part of the primary wi ding and the step down ratio to 6.3vac will be increased and lower the 6.3volt output a bit- I am not sure how much…
I found a manual for the Globe Scout here:

http://www.radiomanual.info/schemi/Surplus_Radioamateur/Globe_Scout_680_user.pdf

I also got the specs for the Globe Scout tranny from Peter Dahl site. (See pics below)
I also took a screen shot of the voltage chart for the Globe scout and it is another pic below.

I also found a manual for the Harvey Wells here:

http://www.museumofyesterday.org/museum/page7_graphics/manuals/harvey_wells_tbs50_manual.pdf

That Harvey Wells had two 5U4s and a 250mA supply. Two 5U4s take 6 amps, by tje way- at 5volts, so it is not an option for you on the Globemaster tranny.

Note that the GlobeMaster was designed for 115volts, by the way.
No reason that your filament resistors would not work. A 25 watt rheostat of a few ohms could allow setting it exactly for your particular setup.
I have more than 125vac on my grid here, so the variac is a great addition and monitoring the voltage could help.
I finally gave in and bought a ferroresonant 500vA transformer for my big tube filaments after comments here made me see it as a good option.
Migjt be overkill to protect an 807 and companions, though…😉
73, Mike




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